Droplet actuator and methods

ABSTRACT

A series of microactuators for manipulating small quantities of liquids, and methods of using these for manipulating liquids, are disclosed. The microactuators are based on the phenomenon of electrowetting and contain no moving parts. The force acting on the liquid is a potential-dependent gradient of adhesion energy between the liquid and a solid insulating surface.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/430,816, filed May 6, 2003 (now U.S. Pat. No. 7,255,780), which is acontinuation of U.S. patent application Ser. No. 09/490,769, filed Jan.14, 2000 (now U.S. Pat. No. 6,565,727), which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/117,002, filed Jan. 25, 1999.

FIELD OF THE INVENTION

This invention relates generally to the fields of laboratory automation,microfabrication and manipulation of small volumes of fluids(microfludics), in such a manner so as to enable rapid dispensing andmanipulation of small isolated volumes of fluids under direct electroniccontrol. More specifically, the invention relates to a method of formingand moving individual droplets of electrically conductive liquid, anddevices for carrying out this method.

BACKGROUND OF THE INVENTION

Miniaturization of assays in analytical biochemistry is a direct resultof the need to collect maximum data from a sample of a limited volume.This miniaturization, in turn, requires methods of rapid and automaticdispensing and manipulation of small volumes of liquids (solvents,reagents, samples etc.) The two methods currently employed for suchmanipulation are, 1) ink jetting and 2) electromigration methods incapillary channels: electroosmosis, elecrophoresis and/or combinationthereof. Both methods suffer poor reproducibility.

Ink jetting is based on dispensing droplets of liquid through a nozzle.Droplet expulsion from the nozzle is effected by a pressure pulse in thereservoir connected to the nozzle. The pressure pulse itself is effectedby an electric signal. The droplets are subsequently deposited on asolid surface opposing the nozzle. The relative position of the nozzleand the surface is controlled by a mechanical device, resulting indeposition of droplets in a desired pattern. Removal of the droplets istypically effected by either washing or spinning (centrifugal forces).

While ink jetting is a dispensing method generally applicable to a widevariety of liquids, the volume of the deposited droplets is not verystable. It depends on both the nature of the liquid being deposited(viscosity, density, vapor pressure, surface tension) and theenvironment in the gap between the surface and the nozzle (temperature,humidity). Ink jetting technology does not provide means to manipulatedroplets after they have been deposited on the surface, except forremoving them.

Electromigration methods are based on mobility of ions in liquids whenelectric current is passed through the liquids. Because different ionshave different mobilities in the electric field, the composition ofliquid being manipulated generally changes as it is being transported.While this feature of electromigration methods is useful for analyticalpurposes, because it allows physical separation of components ofmixtures, it is undesirable in general micromanipulation techniques.

Additionally, the need to pass electrical current through the liquidresults in heating of the liquid, which may cause undesirable chemicalreactions or even boiling. To avoid this, the electrical conductivitiesof all liquids in the system are kept low, limiting the applicability ofelectromigration methods.

The need to pass electrical current through the liquid also requiresthat the control electrodes be electrically connected through anuninterrupted body of conductive liquid. This requirement additionallycomplicates precision dispensing and results in ineffective use ofreagents, because the metered doses of a liquid are isolated from acontinuous flow of that liquid from one electrode to another.

Additionally, ions present in the liquid alter the electric field inthat liquid. Therefore, changes in ionic composition in the liquid beingmanipulated result in variations in resultant distribution of flow andmaterial for the same sequence of control electrical signals.

Finally, the devices for carrying out the electromigration methods haveconnected channels (capillaries), which are used to define liquid flowpaths in the device. Because the sizes of these capillaries andconnections among them are optimized for certain types of manipulations,and also for certain types of liquids, these devices are veryspecialized.

SUMMARY OF THE INVENTION

The present invention provides microchip laboratory systems and methodsof using these systems so that complex chemical and biochemicalprocedures can be conducted on a microchip under electronic control.

The microchip laboratory system comprises a material handling devicethat transports liquid in the form of individual droplets positionedbetween two substantially parallel, Hat surfaces. Optional devices forforming the droplets are also provided.

The formation and movement of droplets are precisely controlled byplurality of electric fields across the gap between the two surfaces.These fields are controlled by applying voltages to plurality ofelectrodes positioned on the opposite sides of the gap. The electrodesare substantially planar and positioned on the surfaces facing the gap.At least some of the electrodes are electrically insulated from theliquid in the gap.

The gap is filled with a filler fluid substantially immiscible with theliquids which are to be manipulated. The filler fluid is substantiallynon-conductive. The wetting properties of the surfaces facing inside thegap are controlled, by the choice of materials contacting the liquids orchemical modification of these materials, so that at least one of thesesurfaces is preferentially wettable by the filler fluid rather than anyof the liquids which are to be manipulated.

The operating principle of the devices is known as electrowetting. If adroplet of polar conductive liquid is placed on a hydrophobic surface,application of electric potential across the liquid-solid interfacereduces the contact angle, effectively converting the surface into morehydrophilic. According to the present invention, the electric fieldseffecting the hydrophobic-hydrophilic conversion are controlled byapplying an electrical potential to electrodes arranged as an array onat least one side of the gap. The electrodes on the other side mayor maynot be arranged in a similar array; in the preferred embodiment, thereis array of electrodes only on one side of the gap, white the other hasonly one large electrode covering substantially the entire area of thedevice.

At least on one side of the gap, the electrodes are coated with aninsulator. The insulator material is chosen so that it is chemicallyresistant to the liquids to be manipulated in the device, as well as thefiller fluid. By applying an electrical potential to an electrode or agroup of electrodes adjacent to an area contacted by polar liquid, thehydrophobic surface on top of these electrodes is converted tohydrophilic and the polar liquid is pulled by the surface tensiongradient (Marangoni effect) so as to maximize the area overlap with thecharged group of electrodes.

By removing an electric potential from an electrode positioned betweenthe extremities of an elongated body of polar liquid, the portion offormerly hydrophilic surface corresponding to that electrode is madehydrophobic. The gradient of surface tension in this case acts toseparate the elongated body of liquid into two separate bodies, eachsurrounded by a phase boundary. Thus, individual droplets of polarliquid can be formed by alternatively applying and removing an electricpotential to electrodes. The droplets can be subsequently repositionedwithin the device as discussed above.

Examples of appropriate coating materials include SiN and BN, depositedby any of the conventional thin-film deposition methods (sputtering,evaporation, or preferably chemical vapor deposition) and parylene™,deposited by pyrolytic process, spin-on glasses (SOGs) and polymercoatings (polyimide s, polymethylmetacrylates and their copolymers,etc.), dipand spray-deposited polymer coatings, as well as polymer films(Teflon™, polyimides etc.) applied by lamination.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 Cross-section of a planar electrowetting actuator according tothe invention

22—top wafer

24—bottom wafer

26—liquid droplet

28 a—bottom hydrophobic insulating coating

28 b—top hydrophobic insulating coating

30—filler fluid

32 a—bottom control electrodes

32 b—top control electrodes

FIG. 2 Pump assembly

FIG. 3 Drop meter

34—contact pad

36—cutoff electrode

FIG. 4 Active reservoir

38—hydrophobic rim

40—reservoir electrodes

FIG. 5 Array

42 a—transport lines

42 b—test areas

FIG. 6 Vortexer

44—sectorial electrode

FIG. 7 Zero-dead-volume valve

62—gate electrode

64 a—first supply line

64 b—second supply line

64 e—common line

FIG. 8 Decade dilutor

46—diluent line

48—reagent supply line

50—vortexer

52—undiluted reagent outlet

54—first stage outlet

56—second stage outlet

58—third stage outlet

60—fourth stage outlet

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, there is provided a chamber filled with afluid, with flat electrodes 32 a,b on opposite surfaces (FIG. 1) Thechamber is formed by the top 22 and the bottom 24 wafers. Themanipulated liquid is presented in the form of droplets 26. The fluid 30filling the chamber should be immiscible with the liquid that is to bemanipulated, and be less polar than that liquid. For example, if liquid26 is an aqueous solution, the filling fluid 30 may be air, benzene, ora silicone oil. The electrodes have electrical connections allowing anoutside control circuit to change their potentials individually or ingroups. At least some of the electrodes have insulating, hydrophobiccoating 28 a,b separating them from the inside of the chamber, and thevoltage is applied in such a manner that no DC voltage difference isapplied to any two non-insulated electrodes.

Example 1—-A Pump

The linear arrangement of electrodes shown in FIG. 2 is an integralpump. A droplet of polar liquid, or a streak: of several electrodelengths, can be moved along by applying a wetting potential to anelectrode on one side of it and removing the wetting potential from thelast electrode under the other side of the streak. To aid the effect ofelectrowetting, in moving liquid from one electrode to another, in apreferred embodiment the gap separating two adjacent electrodes is notstraight. Preferably, it has either sawtooth or meander shape,preferably with rounded corners. The depths and widths of theinterdigitated features of the—adjacent electrodes are preferably chosenso as to promote moving liquid from one electrode to another when thevoltage is applied to the latter electrode, as shown in FIG. 2 a-c. Theinitial position of the droplet 26 is shown in FIG. 2 a. The hatching ofan electrode 32 adjacent to the position of the droplet indicates thatthat electrode is connected to a voltage source. The droplet 26 moves(FIG. 2 b) so as to align itself with the electric field of thatelectrode (FIG. 2 c).

Example 2—A Drop Meter

As a convenient interface between a microfluidics device operating insubnanoliter to microliter range of volumes with the outside world, adrop meter is provided. The drop meter comprises an arrangement controlpads on one side of the chamber (FIG. 3 a), The contact pad 34 is eitherhydrophilic due to material it is made of, or due to a surfacetreatment, or made hydrophilic by applying a wetting potential to anunderlying electrode. The other two control pads have electrodes underthe hydrophobic surface,

To operate the drop meter, a wetting potential is first applied to thecutoff electrode 36 and the control electrode 32. As a result of this,the liquid which has covered the surface of the contact pad 34 spreadsover the other two pads, 32 and 36 (FIG. 3 b-d). Consequently, thewetting potential is removed from the cutoff electrode 36, making ithydrophobic again, Part of the liquid moves back to the contact pad 34,and is replaced on the cutoff electrode 36 with the filling fluid (FIG.3 e-f). As a result, an isolated droplet of liquid (26. FIG. 3 g) isformed on the control electrode 32. The size of the droplet isdetermined by the area of the control electrode 32 and the distancebetween the two surfaces forming the working chamber of the device,

Example 3—An Active Reservoir

A reagent solution may be stored in an active reservoir in a sealeddevice and delivered under electronic control to a reaction site. Anexample of such reservoir is shown in FIG. 4. The delivery is effectedby applying the wetting potential to the first electrode 32 of thetransport line and removing the potential sequentially from thereservoir electrodes 40, for example beginning from the comer(s)furthermost from the transport line. To allow for long storage of thedevices with power off, the coating within the reservoir area is onlymoderately hydrophobic, and the rim 38 around that area is extremelyhydrophobic. The polar liquid not spill beyond the rim 38, allowing longshelf life of the device.

Example 4—An Array

Droplets can be moved by of ectrowetting microactuators in more than onedirection. The array shown in FIG. 5 comprises test areas 42 b (hatched)and transport lines 42 a (open). Reagents are supplied through externaltransport lines, shown (broken) in the top part of the drawing. Wash andwaste lines are arranged similarly. The sources of the reagents may bereagent reservoirs as shown in FIG. 4, drop meters as in FIG. 3, orintegral dilution devices such as shown in FIGS. 6,8. In a preferredembodiment, the test pad electrodes are transparent, for example made ofindium tin oxide (ITO) or a thin, transparent metal film, to allow foroptical detection of molecules immobilized on the pad or trapped in thedroplet.

Such an array has utility as a system for parallel synthesis of manydifferent reagents. Both solid-phase synthesis of immobilized compoundsand liquid-phase synthesis using immobilized reagents, resins andcatalysts are possible. Another use of such an array is a fractioncollector for capillary electrophoresis or similar separation methods,whereby each fraction is isolated by a drop meter (similar to that shownin FIG. 3) and placed on its individual pad 32. This will allow longsignal accumulation time for optical and radioactive detection methodsand therefore improve sensitivity of analysis.

Important features of the electrodes in an array are the width of thegap between the electrodes and the shape of the electrode outline. Toavoid accidental mixing of droplets on the test pads, the gapsseparating those are straight and relatively wide. On the other hand,the electrodes in the transport lines preferably have interdigitatedsawtooth or meander outlines. The gaps between the test pad electrodesand transport line electrodes are also preferably of the meander orsawtooth types.

Example 5—A Mixer/Vortexer

For controlled mixing of solutions, an integral mixer/vortexer isprovided (FIG. 6). It comprises a circular arrangement of sectorialelectrodes 44, some of which have transport line electrodes adjacent to,them. The necessary number of the sectors is fitted with each solutionto be mixed by consecutively applying the wetting potential. to therespective electrodes. The sectors initially filled with differentsolutions are preferably isolated from each other by the interspersedsectors with filler fluid. Then the potentials on the transport linesare removed, and those on sectorial electrodes are rearranged so as tobring the solutions into contact. The mixing action is achieved bysimultaneous removal of filler Fluid from some of the sectors andfilling other sectors with the filler fluid. In particular, vortexeraction is achieved if this is done in a sequential fashion around thecircle.

Alternative configurations of electrodes are possible for achieving thesame goat of assisting in mixing solutions. For example, some of thesectors in an arrangement similar to that shown in FIG. 6 could be madenarrower and longer than the other sectors.

Example 6—A Zero-Dead-Volume Valve

To rapidly exchange solutions contacting a particular pad in an array, azero-dead-volume valve is provided. An example of electrodeconfiguration for this application is shown in FIG. 8. Supply lines 64 aand 64 b are connected to the line 64 c through gate electrode 62.Either of the supply lines is operated in the manner described inExample 1, white wetting potential is applied to the gate electrode.Removal of the wetting potential from the gate electrode 62 allows tomove one of the solutions back up its supply line before the other istransported down its respective line. This arrangement has utility, forexample, in systems for determination of reaction kinetic constants.

Example 7—A Decade Dilutor

A group of mixer/vortexers such as that shown in FIG. 6 can be used,complete with piping, for serial dilutions of reagents. An example of adecade dilutor with five decades is shown in FIG. 8. Each mixer in thedecade dilutor is operated in the manner described in the Example 5.Undiluted solution is passed directly through to the line 52; diluted 10times, down the line 54, and also to the next mixer 50; from there,solution diluted 100 times is passed both down the line 56 and to thenext mixer 50 and so forth.

Such dilutors have utility, for example, as elements of a system fordetermination of binding constants of labeled reagents in solution tothose immobilized on test pads of an array (similar to that shown inFIG. 5). While the present invention has been described in terms ofparticular embodiments, it should be understood that the presentinvention lies in the application of the electrowetting liquidpropulsion principle to forming and manipulating discrete droplets ofliquids rather than a particular structure or configuration of thedevice. It will be obvious to those skilled in the art that a variety ofelectrode configurations and arrangements can be substituted for thosedescribed in the Examples without departing from the scope of thepresent invention. In particular, the dimensions in the figures shouldbe understood only as illustrative examples rather than set dimensionsdefining the scope of the present invention.

That which is claim is:
 1. A device comprising: (a) a first surface anda second surface separated from one another to form a gap between thefirst and second surfaces; (b) plurality of electrodes amounted on thefirst and second surfaces and facing the gap; (c) an insulatorseparating at least a subset of the electrodes from the gap; and (d) adrop meter comprising a contact pad, a cutoff electrode, and a controlelectrode, wherein the contact pad is adjacent to the cutoff electrode,and the cutoff electrode is adjacent to the control electrode.
 2. Thedevice of claim 1 wherein the contact pad comprises an underlyingelectrode.
 3. The device of claim 1 wherein the contact pad comprises ahydrophilic surface.
 4. The device of claim 1 wherein the contact padhas a hydrophilic character imparted by a hydrophilic surface treatment.5. The device of claim 1 wherein the contact pad comprises a hydrophilicmaterial.
 6. The device of claim 1 wherein the plurality of electrodescomprises electrodes coupled by electrical connections to a controlcircuit.
 7. The device of claim 1 wherein the plurality of electrodescomprises electrodes comprising a hydrophilic surface.
 8. The device ofclaim 1 wherein the plurality of electrodes comprises a two-dimensionalarray of electrodes.
 9. The device of claim 1 wherein the plurality ofelectrodes comprises a linear arrangement of electrodes.
 10. The deviceof claim 1 wherein the plurality of electrodes comprises atwo-dimensional array of electrodes on one of the surfaces and theelectrodes on such surface are coated with an insulator.
 11. The deviceof claim 1 wherein the plurality of electrodes comprises a singleelectrode on one of the surfaces.
 12. The device of claim 1 wherein theplurality of electrodes comprises an array of electrodes on the firstsurface and a single large electrode on the second surface.
 13. Thedevice of claim 1 wherein the plurality of electrodes comprisessubstantially planar electrodes.
 14. The device of claim 1 wherein theplurality of electrodes comprises one or more substantially squareelectrodes.
 15. The device of claim 1 wherein the plurality ofelectrodes comprises a path or two-dimensional array of one or moresubstantially square electrodes.
 16. The device of claim 1 wherein theplurality of electrodes comprises one or more substantially triangularelectrodes.
 17. The device of claim 1 wherein the gap comprises a fillerfluid therein.
 18. The device of claim 1 further comprising: (a) afiller fluid in the gap; and (b) a droplet in the filter fluid.
 19. Thedevice of claim 1 further comprising: (a) a filler fluid in the gap; and(b) a polar droplet in the filler fluid.
 20. The device of claim 1wherein the gap comprises: (a) a filler fluid in the gap; and (b) adroplet in the filler fluid; wherein at least one of the surfaces ispreferentially wettable by the filler fluid relative to the droplet. 21.The device of claim 1 wherein the first and second surfaces are arrangedto expose to the gap substantially parallel, flat surfaces.
 22. Thedevice of claim 1 comprising means for supplying voltage to theelectrodes.
 23. The device of claim 1 comprising means for generating anelectric potential to the electrodes.
 24. The device of claim 1 whereinthe insulator is hydrophobic.
 25. The device of claim 1, wherein theinsulator comprises a hydrophobic coating.
 26. The device of claim 1wherein: (a) the insulator comprises a hydrophobic surface; and (b) anelectric field generated by one or more of the electrodes supplied withan electric potential causes a portion of the hydrophobic surface totake on a hydrophilic character.
 27. The device of claim 1 wherein: (a)the insulator comprises a hydrophobic surface; (b) the gap comprises afiller fluid; (c) the filler fluid comprises an immiscible droplettherein adjacent to an electrode; and (d) the immiscible droplet has acontact angle relative to the surface comprising the electrode, whichcontact angle is increased relative to the contact angle of theimmiscible droplet in the presence of an electric potential.
 28. Adevice comprising: (a) two surfaces separated to form a gap; (b) atwo-dimensional array and/or linear path of electrodes positioned toproject an electric field across the gap; c) one or more electrodesexposed to the gap and not coated with an insulator material.
 29. Adevice comprising: (a) two surfaces separated to form a gap; (b) atwo-dimensional array and/or linear path electrodes positioned on one ofthe surfaces and coated with an insulator material; (c) one or moreelectrodes exposed to the gap and not coated with an insulator material.30. A device comprising: (a) two surfaces separated to form a gap; (b)an array and/or linear path of electrodes positioned on one of thesurfaces and coated with an insulator material; (c) a correspondingarray and/or linear path positioned on the other of the surfacescomprising electrodes not coated with an insulator material.
 31. Amethod of forming a droplet, the method comprising: (a) overlapping adroplet with a charged group of electrodes; (b) removing a potentialfrom an intermediate electrode of the charged group of electrodes toseparate the droplet into separate bodies.
 32. The method of claim 31wherein the droplet comprises an elongated body overlapping the chargedgroup of electrodes.
 33. The method of claim 32 wherein the removingstep comprises removing a potential from an electrode positioned betweenextremities of the elongated body.
 34. The method of claim 31 whereinthe overlapping step is accomplished by applying an electric potentialto a group of electrodes to yield the charged group of electrodes. 35.The method of claim 31 wherein the charged group of electrodes comprisesa polymer coating thereon.
 36. The method of claim. 31 wherein thecharged group of electrodes comprises a TEFLON® fluoropolymer coatingthereon.
 37. The method of claim 31 wherein the charged group ofelectrodes comprises a parylene coating thereon.
 38. The method of claim31 wherein the droplet is formed as part of a biochemical procedure. 39.The method of claim 31 wherein the droplet is formed as part of achemical procedure.
 40. A method of dispensing a droplet, the methodcomprising: (a) providing substrate comprising a hydrophilic contact padadjacent to a cutoff electrode adjacent to a control electrode; (b)providing a liquid on the control pad; (c) activating the control pad,the cutoff electrode and the control electrode to spread the liquid overthe control pad, the cutoff electrode and the control electrode; (d)deactivating the cutoff electrode to yield a droplet on the controlelectrode.
 41. The method of claim 40 wherein the hydrophilic contactpad comprises a hydrophobic surface treatment.
 42. The method of claim40 wherein the hydrophilic contact pad is made hydrophilic by activationof an underlying electrode.
 43. The method of claim 40 wherein thedroplet is dispensed as part of a biochemical procedure.
 44. The methodof claim 40 wherein the droplet is dispensed as part of a chemicalprocedure.
 45. A method of forming a droplet, the method comprisingseparating the droplet into separate bodies using a group of electrodes,at least a portion of which electrodes are coated with a hydrophobiccoating.
 46. The method of claim 45 wherein the hydrophobic coatingcomprises a TEFLON® polymer coating.
 47. The method of claim 45 whereinthe hydrophobic coating comprises a parylene coating.
 48. A method oftransporting a droplet, the method comprising: (a) providing a surfacecomprising a first electrode and a second electrode; (b) providing thefirst electrode with a wetting potential causing a droplet to bepositioned on the first electrode; removing the wetting potential fromthe first electrode and applying wetting potential to the secondelectrode, causing the droplet to move from the first electrode to thesecond electrode.
 49. The method of claim 48 wherein the first andsecond electrodes are coated with lipophilic coating.
 50. The method ofclaim 49 wherein the lipophilic coating comprises a TEFLON®fluoropolymer.
 51. The method of claim 49 wherein the lipophilic coatingcomprises a parylene coating.
 52. A method of transporting an elongateddroplet, the method comprising. (a) providing the droplet on a surfacehaving an electrode-mediated wetting potential; (b) applying a wettingpotential to an electrode at one end of the elongated droplet andremoving a wetting potential from another end of the elongated droplet,thereby causing the droplet to be transported.